use Perl

• scale linear then log (Score:0)

  by slanning (5049) on 2008.09.03 10:30 (#64731) Homepage Journal

  Maybe I'm misunderstanding, but I think you want to first scale logarithmically: log($val) if it's log base 'e'. Then linearly: so find the max value in the logged sequence, then multiply each value by 255 and divide by the max. Here's a kind of generic script, if I got it right:

  #!/usr/bin/perl
  # convert @VALS to a log scale base $LOG_BASE
  # and scaled linearly to $SCALE_MAX
  
  use strict;
  use warnings;
  use List::Util qw(max);
  
  my $SCALE_MAX = 1000;
  my $LOG_BASE = 10;
  my @VALS = (1, 10, 100, 1000);
  
  main();
  
  sub main {
      my @logvals = map { logN($_, $LOG_BASE) } @VALS;
      my $maxval = max @logvals;
  
      foreach my $val (@logvals) {
          print $val * $SCALE_MAX / $maxval, $/;
      }
  }
  
  sub logN {
      my ($val, $N) = @_;
      return ($N eq 'e') ? log($val) : log($val) / log($N);
  }

  Reply to This

  • Re: (Score:1)

    by veryrusty (8826)

    Need to scale linearly first in the case the minimum value is less than 1 before taking logs. (Also, $SCALE_MAX should be 255)

    Instead, if you simplify the mathematics (and you know the minimum and maximum values), calculate the logarithmic 'scaling factor'

    $scale = log( $maximum ) / 255;

    Then apply logarithmic scaling to each data element

    map { int( log( $_ - $minimum + 1 ) / $scale ) } @VALS;

    So that the minimum value scales to zero, maximum value scales to 255. Adjust the linear scale in the log calculatio